1. Field of the Invention
This invention relates to a sports racket frame construction, especially a tennis racket frame constructed from a composite structure. The sports racket frame has a head portion and a handle portion constructed from a rolled up hollow tube which is constructed from a plurality of strips of unidirectional high strength composite material comprising unidirectional high strength fibers impregnated with thermoplastic resin, such as in unidirectional composite thermoplastic tape. The unidirection of the fibers in at least two strips are at different angles. The hollow tube is formed into the shape of the racket, placed in a mold together with yoke components constructed of the same composite material for consolidation of the hollow tube to form the frame.
2. Description of the Prior Art
Sports rackets such as especially tennis rackets, as well as squash rackets, racquetball rackets, badminton rackets, etc., have been constructed of numerous materials by using various techniques in an effort to obtain superior strength-to-weight and playability characteristics. Originally, tennis rackets, for example, utilized a frame of solid wood with various species of wood being used depending upon the characteristics of the selected wood as determined by the manufacturer and user. Subsequently, laminated wood tennis racket frames were manufactured of various wood materials and these structures sometimes included reinforcements. Laminated wood construction provided improvements over solid wood tennis racket frames but also included undesirable characteristics such as lack of uniformity, low tensile and compressive strength characteristics and relatively short life due to the tendency of the laminations to separate and for other reasons. When the laminated wood structure deteriorates due to warpage, scuffing and the like, the stiffness and strength properties of the frame are reduced as well as the color and appearance characteristics. In addition, it is difficult to manufacture a laminated wood tennis racket frame having uniform weight characteristics, balance characteristics and performance characteristics.
As a result of seeking sports rackets, especially tennis and racquetball rackets, having beneficial characteristics, metal tennis racket frames have been manufactured which usually include a tubular frame or a frame of some other cross section such as cylindrical, channel shaped, I-beam shape and the like usually constructed from extrusions of aluminum, magnesium, steel alloys or similar metallic materials which solve some of the problems of wooden rackets, but introduced their own problems of weight limitations which were encountered when endeavoring to achieve desired performance characteristics. Additionally, metal racket frames transmit vibration and shock forces to the player which contribute to player fatigue and tennis elbow injury. Also, tennis rackets with metal frames produce sounds that are sometimes found objectionable to players and spectators. Further, performance is adversely affected by a trampoline effect produced by metal tennis racket frames.
More recent efforts in the development of sports rackets have involved the use of resin materials reinforced with high strength fibers, particularly carbon fibers which are used to make the so-called graphite frame. The standard method of producing composite material racket frames is by using thermoset prepregs. These come in the form of unidirectional sheets, woven fabrics, or braided tubes.
In a conventional method, fiber-impregnated resin tennis racket frames are made starting with sheets of a prepreg material composed of aligned carbon fibers impregnated with uncured thermosetting resin, e.g., a B-stage epoxy resin. An uncured thermosetting resin is neither liquid nor solid, and creates a prepreg which is very drapable, soft and formable. It is also very tacky so that plies stick to each other and can be rolled up and packed in a forming mold for curing and forming.
According to this method, the sheets are cut at specific angles and rolled into the shape of an elongated tube. Successive layers are oriented at alternating angles, in order to impart directional rigidity and strength to the finished frame. Usually, the tube is formed on a cylindrical mandrel over an inflatable bladder or hand rolled. The mandrel is then withdrawn and the tube, still containing the bladder, is packed into a mold in the shape of the frame. In the mold, the bladder is inflated forcing the prepreg tube to conform to the shape of the mold, and the prepreg is the heat cured and hardened.
The foregoing method has the advantage of being able to produce high quality racket frames, but has numerous disadvantages. Since thermosetting resins are very tacky at room temperature, a significant amount of labor is required in forming the rolled tube and in packing it in the mold. Also, when using this method the prepreg tubes need to be stored in a cool environment. Further, the cured frame ofttimes has imperfections when then require significant work in hand finishing after removal from the mold. Furthermore, the use of thermosetting resins has come under recent criticism since such resins cannot be recycled and the discarded product made of such resins has become a serious disposal problem.
Thus, there has recently been intense interest in developing a reinforced thermoplastic resin racket which does not suffer the manufacturing difficulties of a thermosetting resin since thermoplastic resins can be reheated and recycled. Further, thermoplastic resins offer the possibility for even better strength-to-weight and playability characteristics. However, prior to the present invention, it has been generally recognized that the above-described process cannot, as a practical matter, be used with thermoplastics. In contrast to uncured epoxy prepregs, which at room temperature are drapable, a thermoplastic prepreg would be very hard, stiff and "boardy". In order to use the above process it would thus be necessary to heat the resin to a very high temperature first to form the tube, and again in order to pack the tube in the mold, making it difficult for workers to handle in racket forming operations. Also, a thermoplastic prepreg would possess no tack, making it difficult to form a tubular layup using multiple layers. Additionally, thermoplastic materials have a relatively small window near the melting point before they start to flow. Even assuming that the prepreg could be heated to the softening point without melting the resin, it would be difficult to maintain constant temperature during processing. As a result, attempts to manufacture a high strength sports racket utilizing a thermoplastic resin have been very limited.
There have been several proposals made, and several tennis rackets introduced to the market, which are made of injection molded, fiber-reinforced thermoplastics. However, these do not employ the same process used to make thermoset rackets, and do not have the same fiber structure. Instead, injection molded thermoplastic rackets are formed using a mixture of resin and short length fibers, which is injected into a racket mold. The fibers are disbursed through the resin in a random orientation to produce a material with isotropic properties. This has the advantage of simplifying the racket-forming process, in that the number of manual steps is reduced. However, injection molding processes posses the major limitation that, because the fibers pass through an injection nozzle, they cannot exceed about one-half inch. Inherently then, the reinforcing fibers are much shorter than in prepreg thermoset resin processes, and do not produce the same strength and overall racket stiffness as in the case of the longer fibers present in a thermoset racket. Moreover, known thermoplastic processes have the disadvantage that the fiber orientation cannot be controlled.
U.S. Pat. No. 5,176,868, assigned to Prince Manufacturing, Inc., which is directed to the Prince "VORTEX" tennis racket, utilizes the combination of a plurality of reinforcing fibers with a thermoplastic resin and is composed of commingled thermoplastic filaments and reinforcing fibers which are braided to form a flexible sleeve with the flexible sleeve being placed in a mold having a cavity in the shape of a tennis racket frame with a bladder being disposed within the sleeve. The mold is closed and heated until the thermoplastic melts and the bladder is pressurized so that the sleeve conforms with the mold and, after cooling is removed from the mold as a tennis racket frame.
This process of the Prince U.S. Pat. No. 5,176,868, however, has several drawbacks. Initially, there is the necessity for a separate braiding operation. Next, the bias of the fiber orientation is limited by the braiding to about -15.degree. to +15.degree., thus limiting the reinforcing capabilities of the fibers. Further, the injection of the dry thermoplastic resin into the braided fibers inherently results in nonuniformity of the resin impregnation. Also, the bulkiness of the impregnated braid makes it difficult to place in the forming mold because individual fibers tend to stick out from the tows. In addition, the nature of the braiding system prevents selective variations in the cross-sectional thickness at desired locations in the racket frame, as well as variations in the specific strength characteristics which can be designed into a rolled prepreg for selected locations in the racket frame.
U.S. Pat. No. 4,070,020, issued in 1978, discloses the formation of a tennis racket utilizing a rolled tube of graphite fiber and thermosetting resin. The rolled tube is made using a veiling strip, cellulose sleeve and a core of an expandable material with the plies of resin impregnated unidirection graphite fiber sheets, veiling strip, cellulose sleeve and core being placed in a mold and heated to curing temperature. When so heated, the core is caused to expand thus producing an internal pressure within the tube in the mold cavity so that the mold forms the tube into the shape of a racket frame, at the same time that the thermosetting resin is cured. While the patent discloses that the resin material could be a thermoplastic, there is no disclosure as to how the thermoplastic sheets could be rolled into the form disclosed or how the aforesaid recognized problems associated with making a racket using thermoplastic resins are solved.
The following additional U.S. patents are relevant to this invention.
______________________________________ 3,915,783 4,128,963 4,874,563 4,023,799 4,194,738 4,957,883 4,028,477 4,212,461 4,983,242 4,070,019 4,294,787 5,013,507 4,114,880 4,770,915 5,076,872 4,124,670 4,770,929 5,198,058 4,129,634 4,871,491 ______________________________________
None of the prior patents, however, disclose a sports racket frame constructed from a rolled up elongated hollow tube made from unidirectional thermoplastic composite tape in which a plurality of strips of unidirectional high strength fibers impregnated with thermoplastic resin are rolled into the hollow tube.